U.S. Pat. No. 4,616,948, the entire disclosure of which is incorporated herein by reference, discloses a friction coupling for locking a shaft relative to a hub. The friction coupling comprises a radially deformable inner sleeve and a radially deformable outer sleeve, and an annular piston which is axially movable relative to the inner sleeve and the outer sleeve. The annular piston has a conical surface, which cooperates with the outer sleeve and which is arranged to deform the inner sleeve and/or the outer sleeve when the annular piston is moved relative to the inner sleeve and the outer sleeve, thus applying a contact force between, on the one hand, the inner sleeve and the shaft and, on the other hand, the outer sleeve and the hub. The contact force produces a frictional force, which causes the shaft and the hub to be locked relative to each other. The friction coupling further comprises an actuating pressure chamber, which when pressurised produces a movement of the annular piston for locking the coupling, and a deactuating pressure chamber, which when pressurised produces a movement of the annular piston for unlocking the coupling. The actuating pressure chamber and the deactuating pressure chamber are located, seen in the axial direction, on opposite sides of the annular piston.
Other examples of friction couplings of similar type are known from U.S. Pat. No. 4,859,106, U.S. Pat. No. 5,149,220 and U.S. Pat. No. 5,156,480, the entire disclosure of which is incorporated herein by reference.
A drawback of the above-mentioned friction coupling is that if connecting means for feeding of compressed fluid to the actuating pressure chamber and the deactuating pressure chamber are to be arranged on the same side of the coupling, seen in the axial direction, a duct has to be provided through the inner sleeve or the outer sleeve, which makes it difficult to reduce the radial extension of the friction coupling, thus making the friction coupling radially unwieldy, and also increasing its weight.
Another drawback is that the presence of the duct causes problems of strength, which have to be compensated for by an increase of the material thickness of the coupling or by load limits on the coupling.
Yet another drawback is that the above-mentioned friction coupling requires relatively heavy forces for actuation or deactuation. Since the piston surface facing the actuating pressure chamber and the deactuating pressure chamber is relatively small, a very high pressure is required, for instance, often a pressure of up to 1000 bar for assembling and of up to 1200 bar for disassembling. This puts great demands on the equipment needed for actuation and deactuation, which results in higher costs of the equipment.